Magnetic Resonance Imaging (MRI) is a medical imaging technique most commonly used in radiology to visualize a detailed internal structure and a function of a body of a human being. MRI uses a powerful magnetic field to align nuclear magnetization of usually hydrogen atoms in water in the body. Radio frequency (RF) fields are used to systematically alter the alignment of this magnetization, causing hydrogen nuclei to produce a rotating magnetic field detectable by a scanner. This detected signal can be manipulated by additional magnetic fields to build up enough information to construct an image of the body.
During a MRI procedure, the patient absorbs a portion of a transmitted RF energy, which can result in body tissue heating and other adverse effects, such as alterations in visual, auditory and neural functions. The so-called Specific Absorption Rate (SAR), in watts per kilogram (W/kg), is the RF power absorbed per unit mass of tissue. The SAR is one of the most important parameters related with thermal effects and acts as a guideline for MRI safety.
For an exact determination of local SAR, a spatial distribution of an electric field of the involved RF coil throughout the patient as well as an electric conductivity and permittivity distribution and a mass density throughout the patient is required. In principle, the electric field can be calculated from three spatial components of the magnetic field. A spatial distribution of the electric properties conductivity and permittivity can be determined via Ampere's law known in the art.
No reliable method has been found to determine in vivo the electric field and electric conductivity, and thus, the local SAR. Instead, rough estimations are performed, based on models. These models are based on human anatomy obtained from single individuals in a fixed position. Usually, the body is dissected into sub-cm voxels, each assigned a particular tissue type and thus well-defined electric properties, i.e. electric conductivity σ and permittivity ∈, and mass density ρ. Using a model of the applied RF coil, the electric fields and accordingly local SAR are determined via simulations. This approach is not patient specific with regard to individual anatomy and position. Patient specific models within this framework are not practicable due to the simulation times of several hours. The uncertainties of such models require large safety margins, frequently leading to a potentially unnecessary increase of the repetition time, and thus, the total acquisition time.
An embodiment of such a method is disclosed in WO-2007017779 A2 patent application. The method according to this patent application aims at measuring the body electric properties σ and ∈ via measuring magnetic fields produced by a RF coil and performing simulations. For SAR computation, the measured electric properties are used together with electric fields computed via simulation using patient models and RF coil models. Such simulation is based on the measurement of the magnetic fields.
A drawback of the known system is that the full simulation of coil and patient is very time consuming, in particular at least several hours, and thus not practicable in a clinical setting.